1. Field of the Invention
The invention relates to optical fibers. More particularly, the invention relates to a super-large-effective-area optical fiber that exhibits low loss and that has a broad operation wavelength range.
2. Description of the Related Art
Optical fibers are thin strands of glass or plastic capable of transmitting optical signals, containing relatively large amounts of information, over long distances and with relatively low attenuation. Typically, optical fibers are made by heating and drawing a portion of an optical preform comprising a refractive core region surrounded by a protective cladding region made of glass or other suitable material. Optical fibers drawn from the preform typically are protected further by one or more coatings applied to the cladding region.
Advances in transmission over optical fibers have enabled optical fibers to have enormous bandwidth capabilities. Such bandwidth enables thousands of telephone conversations and hundreds of television channels to be transmitted simultaneously over a hair-thin fiber. Transmission capacity over an optical fiber is increased in wavelength division multiplexing (WDM) systems wherein several channels are multiplexed onto a single fiber, with each channel operating at a different wavelength. However, in WDM systems, nonlinear interactions between channels occur, such as 4-photon mixing, which severely reduces system capacity. This problem has been largely solved by U.S. Pat. No. 5,327,516 (the '516 patent). The '516 patent discloses an optical fiber that reduces these nonlinear interactions by introducing a small amount of chromatic dispersion at the operating wavelengths.
As the number of WDM channels to be transmitted over a single fiber increases, the optical power carried by the optical fiber also increases. As the optical power increases, the nonlinear effects caused by interaction between the channels also increases. Therefore, it is desirable for an optical fiber to provide a small amount of chromatic dispersion to each of the WDM channels in order to reduce the nonlinear interactions between the channels, especially in view of ever-increasing bandwidth demands. However, in order to be able to restore the signal after the transmission link, it is important that the dispersion introduced vary as little as possible amongst the different WDM channels.
Important advances have been made in the quality of the material used in making optical fibers. In 1970, an acceptable loss for glass fiber was in the range of 20 decibels per kilometer (dB/km), whereas today losses are generally about 0.25 dB/km. The theoretical minimum loss for glass fiber is less than 0.15 dB/km, and it occurs at a wavelength of about 1550 nanometers (nm). Dispersion in a glass fiber causes pulse spreading for pulses that include a range of wavelengths, due to the fact that the speed of light in a glass fiber is a function of the transmission wavelength of the light. Pulse broadening is a function of the fiber dispersion, the fiber length and the spectral width of the light source. Dispersion for individual fibers is generally illustrated using a graph (not shown) having dispersion on the vertical axis (in units of picoseconds (ps) per nanometer (nm), or ps/nm) or ps/nm-km (kilometer) and wavelength on the horizontal axis. There can be both positive and negative dispersion, so the vertical axis may range from, for example, −250 to +25 ps/nm km. The wavelength on the horizontal axis at which the dispersion equals zero corresponds to the highest bandwidth for the fiber. However, this wavelength typically does not coincide with the wavelength at which the fiber transmits light with minimum attenuation.
For example, typical first generation single mode fibers generally transmit with minimum attenuation at 1550 nm, whereas dispersion for the same fiber would be approximately zero at 1310 nm. Also, the aforementioned theoretical minimum loss for glass fiber occurs at the transmission wavelength of about 1550 nm. Because Erbium-doped amplifiers, which currently are the most commonly used optical amplifiers for amplifying optical signals carried on a fiber, operate in 1530 to 1565 nm range, the transmission wavelength normally used is 1550 nm. Because dispersion for such a fiber normally will be closest to zero at a wavelength of 1310 nm rather than at the optimum transmission wavelength of 1550 nm, attempts are constantly being made to improve dispersion compensation over the transmission path in order to provide best overall system performance (i.e., low optical loss and low dispersion).
It is desirable to suppress the aforementioned nonlinear optical effects and to reduce attenuation over a broad bandwidth in order to improve the spectral efficiency and reduce the bit-error-rate of wavelength division multiplexing and dense wavelength division multiplexing (WDM/DWDM) optical transmission systems. Super-large effective area (SLA) fibers have been developed to meet these needs. SLA fibers are normally used as transmission fibers and normally have both a positive dispersion and a positive dispersion slope. The large effective areas of these fibers suppress nonlinear effects so that transmission is improved over a broad wavelength range. However, most SLA fibers currently being produced have a cutoff wavelength at approximately 1450 nm, which presents two disadvantages. First, this cutoff wavelength makes single mode operation within the ˜1300 nm wavelength window impossible, which is the wavelength window in which dispersion is minimized for single mode fibers. SONET/SDH transmission at 1310 nm remains popular in metro networks. In addition, longer distance (e.g., greater than 20 km) cable television transmission at 1550 nm could benefit by reducing the threshold for Stimulated Brillouin Scattering (SBS) in SLA fiber. However, the higher cutoff wavelength of current SLA fibers would preclude use of 1310 nm services on the same fiber route, making it less flexible and therefore less likely to be deployed. Finally, a cutoff wavelength of 1450 nm is not optimum for Raman pumping of signals in the S and C bands.
It would be desirable to provide an SLA optical fiber having a lower cutoff wavelength than existing SLA fibers and which has the same or improved transmission properties when compared with those associated with existing SLA fibers, including, for example, reduced nonlinear optical effects and low attenuation over a broad range of wavelengths.